The leading two causes of death listed by the World Health Organization (1998) are coronary heart disease and cerebrovascular disease. Since these diseases are largely triggered by occlusive blood clots, there is a considerable need for safe and effective thrombolytic agents (drugs capable of dissolving clots and restoring blood flow). However, blood clots also perform the essential physiological function of preventing hemorrhage by sealing injured vessels and tissues. This process is called hemostasis and most thrombolytic drugs interfere with hemostasis, and thereby induce bleeding at the sites of injured vessels and tissues.
When an intravascular clot or thrombus forms, blood flow is arrested at that site. Depending on the location in the arterial system, i.e., heart, brain, or leg, such an occlusive clot can trigger a heart attack, stroke, or peripheral gangrene. In the venous circulation the same process can cause thrombophlebitis (deep vein thrombosis) or pulmonary embolism (lung clots). Together, these cardiovascular diseases constitute the leading causes of death and disability in industrialized countries. Since the tendency to clot increases with age and populations are getting older, the incidence of those disorders is increasing worldwide.
As a result, there is a need for safe and effective thrombolytic agents. One such agent is pro-urokinase (pro-UK), a natural plasminogen activator that targets blood clots (see, e.g., Husain et al., U.S. Pat. No. 4,381,346). It is the proenzyme precursor of urokinase (UK). Pro-UK is better adapted to pharmacological use than UK, because it is inert in the blood (being a pro-enzyme) at physiological concentrations in the absence of a clot. Unfortunately, at therapeutic doses, which are significantly larger than naturally occurring concentrations, pro-UK becomes unstable and is readily converted to UK, which is a non-specific plasminogen activator with undesirable side effects.
Mutant forms of pro-UK are described in Liu et al., U.S. Pat. No. 5,472,692. These pro-UK mutants are said to have lower intrinsic activity than pro-UK, induce less fibrinogenolysis and non-specific plasminogen activation than pro-UK, and yet retain the same thrombolytic activity as pro-UK. In other words, the mutant pro-UKs have been described as performing in the same way as pro-UK, but with fewer side effects because they are more stable than pro-UK in blood at therapeutic concentrations.